Major CI cleanup

[pavelzw]

No description provided.

[pavelzw]

The current state of this PR runs the CI on every PR commit for all systems. If we want to limit this depending on if it is a PR or if it is push: main, we can also generate the matrix dynamically, similar to https://github.com/pavelzw/homebrew-pavelzw/blob/daa7f792956b4a0d36e1f5220ba3ee08adaa0294/.github/workflows/bump.yml#L13-L40

[MarcAntoineSchmidtQC]

This is great! Thanks a lot for the cleanup.
Regarding whether we want to run builds on all architecture for every branch/PR, I think the original reason was that MacOS and Windows runners are much more expensive and we don't need it as much. Let's try it with your approach and reevaluate later whether we want to restrict them again.

My main comment is about the frequency at which we run the conda-build CI. I think right now it would basically only run when we merge a PR (if this PR doesn't modify the CI files). I think we should run it more often.

[pavelzw]

I think the original reason was that MacOS and Windows runners are much more expensive

I might be wrong here but are we even paying for the CI resources in this repo since it is open-source? @xhochy